Method and apparatus for damping vibrations in a semiconductor wafer handling arm

ABSTRACT

A method of damping a semiconductor wafer handling arm by attaching a spring and a mass coupled to the spring to form a mass spring system that is tuned to vibrate at a structural resonant frequency of the vibrating wafer handling arm. The spring has temperature insensitive spring characteristics and the mass and spring are constructed of materials that do not outgas or produce contaminants in a semiconductor processing environment. The mass spring system is preferably a cantilever beam spring connected to a high response point on the vibrating arm and oriented to vibrate in a plane perpendicular to the plane of the wafer. The mass is slidably adjustable along the length of the cantilever beam spring to adjust the resonant frequency. Vibration damping of the wafer handling arm is accomplished by the transfer of kinetic energy from the vibrating wafer handling arm to the mass spring system.

This application is a divisional of U.S. Pat. No. 6,761,085, filed Feb.6, 2002, now U.S. Pat. No. 6,761,085 patented Jul. 13, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to transport mechanisms for rapidly moving andpositioning semiconductor wafers. More specifically, this inventionrelates to methods and devices for damping vibrations in a handling armused in such a transport mechanism.

2. Description of Related Art

In modern high speed semiconductor manufacturing operationssemiconductor wafers are moved into position for processing with anautomated transport mechanism. The transport mechanism uses a handlingarm to pick up the wafer and move it into the desired position. One endof the handling arm is connected to the driving portion of the transportmechanism and the other end holds the wafer as it is moved.

One limitation on the speed of processing is the speed with which thehandling arm can position and remove the wafer from the processingstation. The rapid motion necessary to achieve high speed operationexcites vibrations in the handling arm. These vibrations must be allowedto dissipate at least partially before the wafer can be picked up or setdown. If the vibrations are not allowed to dissipate sufficiently,positioning accuracy deteriorates. In addition, excess vibration resultsin particle generation due to impacts between the arm and wafer. Thisintroduces contaminants into the processing chamber.

Existing semiconductor wafer handling arms are not provided withvibration dampers due to the difficulty of finding vibration absorbingmaterials that are suitable for use in semiconductor processingenvironments. Semiconductor processing environments are typicallycharacterized by wide pressure and temperature ranges and normally havevery low contaminant requirements imposed upon them.

Materials commonly used in other applications to absorb vibrationsoutgas at low pressures. This creates a source of contaminants that isunacceptable in the ultra-lean environment needed for semiconductorwafer processing. In addition, the damping properties of conventionaldamping materials vary significantly across the temperature rangerequired to process wafers. This makes them a poor choice for use insuch an environment.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a vibrationdamper for a handling arm that does not use conventional vibrationdamping materials with their associated contaminant risk and whichprovides vibration damping for the handling arm over a wide range ofprocessing environment temperatures.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects, which will be apparent to those skilled inthe art, are achieved in the present invention which is directed to avibration damper for a wafer handling arm that includes a mass and aspring connected between a response point on the wafer handling arm andthe mass to form a mass spring system. The mass spring system has adesired vibration frequency set by the characteristics of the spring andthe mass and that vibration frequency corresponds to a bending modevibration frequency of the wafer handling arm to be damped by thevibration damper.

The spring is preferably a cantilever beam spring, although springs ofother types may be used. In the preferred design, the mass is slidablymounted on an end of the cantilever beam spring to permit adjustment ofthe length of the spring and thereby adjust the vibration frequency forthe mass spring system.

Adjustment is provided through a slot in the cantilever beam spring. Themass slides along the slot to permit adjustment of the vibrationfrequency for the damper. In the most highly preferred embodiment, themass includes a channel sized to match the cantilever beam spring, themass being guided along the cantilever beam spring by engagement betweenthe channel and the cantilever beam spring.

The wafer held by the wafer handling arm defines a plane and thecantilever beam spring and mass are preferably oriented to vibrate outof the plane defined by the wafer handling arm to damp out of planevibrations of the wafer handling arm corresponding to the second out ofplane bending mode for the arm.

The spring is constructed of a material that has substantiallytemperature insensitive spring characteristics over a temperature rangesuitable for semiconductor processing, such as stainless steel. The massis also preferably constructed of stainless steel. The materialsselected for the damper also should not outgas at low pressures found ina semiconductor processing environment.

The present invention also includes the method of damping vibrations ina wafer handling arm. The preferred method includes the steps of:

-   -   selecting a spring constructed of a material suitable for use in        a semiconductor processing environment;    -   selecting a mass constructed of a material suitable for use in a        semiconductor processing environment;    -   positioning the mass on the spring to form a mass spring system        having a desired vibration frequency, the vibration frequency of        the mass spring system corresponding to a bending mode vibration        frequency of the wafer handling arm to be damped by the        vibration damper; and    -   attaching the mass spring system to the wafer handling arm at a        high response point for the bending mode of the wafer handling        arm to be damped by the vibration damper.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a perspective view of a wafer handling arm and a vibrationdamper according to the present invention. A semiconductor wafer isshown in phantom.

FIG. 2 is an exploded perspective view of a vibration damper accordingto the present invention.

FIG. 3 is a time history graph of the out of plane acceleration of anundamped conventional wafer handling arm moving at a constant armvelocity of twelve inches per second showing out of plane freevibrations.

FIG. 4 is a time history graph showing the reduction in out of planeacceleration of the same wafer handling arm measured in FIG. 3, movingat the same speed, when the vibration damper of the present invention isattached as seen in FIG. 1.

FIG. 5 is a time history graph of the out of plane acceleration of anundamped conventional wafer handling arm following a 0.6 second armmotion showing out of plane free vibrations.

FIG. 6 is a time history graph showing the reduction in out of planevibration acceleration of the same wafer handling arm measured in FIG.5, after the same 0.6 second arm motion, when the vibration damper ofthe present invention is attached as seen in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1 to 6 of the drawings in whichlike numerals refer to like features of the invention.

FIG. 1 illustrates a semiconductor wafer handling arm 10 having avibration damper 12 according to the present invention attached to thearm 10. The base 14 of the handling arm 10 is provided with a mount forattachment to the driving portion of the transport mechanism (not shown)and the opposite end 16 is provided with a mechanism that holds thesemiconductor wafer 18 (shown in phantom).

The wafer 18 defines a plane and the vibrations of greatest concern arevibrations due to the second bending mode in the direction perpendicularto that plane. These “out of plane” vibrations cause impacts between thearm and the wafer that produce contaminant particles. The vibrationdamper 12 is attached to the arm at point P, which is a high responsepoint for the second bending mode of the wafer handling arm 10. Thevibration damper operates by transferring kinetic energy from thevibrating handling arm 10 to a mass spring system comprising thevibration damper 12.

FIG. 2 provides an exploded detail view of the vibration damper of thepresent invention. The damper 12 includes a spring 20 with an end mass22. The spring 20 and mass 22 form a mass spring system that vibrates ata frequency determined by the characteristics of the spring and themass. The shape of the spring and the material used to make itprincipally determine the characteristics of the spring. In theembodiment shown, the spring 20 is a cantilever beam type spring, andthe spring characteristics are primarily set by the length, width andthickness of the beam. The spring characteristics are adjustable tochange the resonant frequency of the mass spring system by providing asliding adjustable connection between the mass 22 and the beam spring20. Sliding the mass varies the effective length of the cantilever beamspring.

Although the cantilever-type of spring is preferred, the presentinvention is not limited to cantilever beam springs. Other types ofsprings, including coil springs may be used to form the combined massspring system. The mass spring system simply needs to be capable ofbeing coupled to the arm and have a resonant frequency that allowsvibrational kinetic energy (in the bending mode to be damped) to betransferred from the arm to the mass spring system. Those of skill inthe art will recognize that many types of mass spring systems can beconstructed and oriented in the required way to receive the transferredvibrational kinetic energy. Other bending modes may also be damped byadjusting the resonant frequency and/or orientation of the mass springsystem or by using additional vibration dampers.

Also, adjustment of the resonant frequency of the mass spring system isnot limited to varying the position of a mass or the length of thespring beam. The frequency may be adjusted by changing the amount ofmass attached to the spring or by changing the spring characteristicssuch as by using a thicker or thinner spring beam, making the beam intoa multiple leaf spring and adding or removing spring leaf beams, or byadding coil springs or other types of additional or different shapedsprings. It is also contemplated that a non-adjustable mass springsystem can be used once the desired spring characteristics and mass areknown.

A first end 24 of the cantilever beam spring 20 is provided withmounting holes 26 for attaching the cantilever beam to the handling arm10 at the desired high vibration response point P corresponding to thebending mode of the arm to be damped. The opposite end 28 of thecantilever spring is provided with slot 30. An attachment screw 32extends through opening 34 in the end mass 22 and is threaded into nutplate 36.

The end mass 22 is provided with a channel 38 that corresponds to thewidth of the end 28 of cantilever beam spring 20. The channel 38 alsoreceives the nut plate 36. The nut plate and screw 32 also form a partof the mass in the mass spring system. Moving the end mass 22 towardsthe wafer handling arm 10 increases the resonant frequency of the massspring system. Moving the end mass 22 away from the handling arm 10decreases the resonant frequency of the vibration damper. By adjustingthe position of the end mass, the resonant frequency of the damper canbe tuned to match the dominant structural resonance of the vibratingwafer handling arm.

Preferably the damper is tuned to match the frequency of the handlingarm's second out of plane bending mode and the attachment point for thevibration damper to the handling arm is a high response point for thisbending mode. The point P is selected to efficiently transfervibrational kinetic energy from the wafer handling arm to the massspring system of the vibration damper at the dominant handling armresonant frequency.

The use of the mass spring system for damping eliminates the necessityfor a conventional energy absorbing damping materials with theirattendant outgas and temperature dependency problems. The damper may beconstructed of any elastic material suitable for use in a waferprocessing environment. In the preferred embodiment illustrated, thedevice is constructed entirely of 304 stainless steel.

FIG. 3 is a graph illustrating the vibrations encountered in aconventional undamped handling arm such as a handling arm shown in FIG.1 without the vibration damper 12 attached. The measurements shown inFIG. 3 were taken with the arm moving at a constant velocity of 12inches per second. FIG. 4 illustrates the vibration reduction achievedwhen the present invention is attached. Vibrational energy has beentransferred to the mass spring system reducing the amplitude of thevibrations of the handling arm 10. FIGS. 5 and 6 show the undamped anddamped responses, respectively, for a sudden arm motion lasting 0.6seconds.

As can be seen by comparing FIGS. 4 and 6 to FIGS. 3 and 5, thevibration reduction with the present invention attached is significantand shows a reduction of vibration levels by at least 50%.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1. A method of damping vibrations in a wafer handling arm for handling asemiconductor wafer, the wafer handling arm defining a plane and themethod comprising the steps of: constructing a cantilever beam spring ofa contaminant-free material suitable for use in a low-contaminantsemiconductor wafer processing environment; constructing a mass of acontaminant-free material suitable for use in a low-contaminantsemiconductor wafer processing environment; positioning the mass on thecantilever beam spring to form a mass spring system having a desiredvibration frequency in an out of plane direction, the vibrationfrequency of the mass spring system corresponding to an out of planebending mode vibration frequency of the wafer handling arm to be damped;and attaching the mass spring system to the wafer handling arm at a highresponse point for the out of plane bending mode of the wafer handlingarm to be damped to provide contaminant-free damping by the transfer ofkinetic energy from the wafer handling arm to the mass spring system andprevent contaminant-producing impacts between the wafer handling arm andthe wafer.
 2. The method of damping vibrations in a wafer handling armaccording to claim 1 wherein the step of positioning the mass on thespring includes the step of adjusting the location of the mass relativeto the spring after the mass spring system has been attached to thewafer handling arm to adjust the vibration frequency of the mass springsystem.
 3. The method of damping vibrations in a wafer handling armaccording to claim 1 wherein the step of constructing a spring includesconstructing the spring of a material having substantially temperatureinsensitive spring characteristics over a temperature range suitable forsemiconductor processing.
 4. The method of damping vibrations in a waferhandling arm according to claim 3 wherein the step of constructing aspring includes constructing the spring of stainless steel.
 5. Themethod of damping vibrations in a wafer handling arm according to claim1 wherein the step of selecting a spring comprises selecting a springconstructed of a material that does not outgas in a semiconductorprocessing environment.